The major thrust of this application is to understand the molecular details of how various cancer therapeutic modalities affect DNA. We have developed a simplified general approach to the study of the physicochemical and biochemical consequences of structural lesions in DNA, using DNA oligomers containing a specific structural lesion at a single site at 100% frequency. We will use this general approach for the study of several important DNA lesions including: (1) the lesion introduced through misincorporation of DHPG, an exciting new anti-viral agent, (2) the hydroxymethal damage to DNA, and (3) the 5-azacytidine lesion introduced by misincorporation. We will study biochemical effects on processes related to DNA replication such as chain elongation, ligation, and movement of a complete replication fork past a lesion site. Using NMR spectroscopy, we will investigate the physicochemical consequences of these lesions. Through construction of computer graphic models we will investigate the detailed structural consequences of lesions in DNA.